80.Correlation of opposites. There are several Herakleitean fragments which form a class by themselves, and are among the most striking of the utterances that have come down to us. These assert in the most downright way the identity of various things usually regarded as opposites. The clue to their meaning is to be found in the account already given of the assertion that day and night are one. We have seen that Herakleitos meant, not that day was night, or night was day, but that they were two sides of the same process: namely, the oscillation of the "measures" of fire and water, and that neither would be possible without the other. Any explanation that can be given of night will also be an explanation of day, and vice versa; for it will be an account of what is common to both, and manifests itself now as one and now as the other. Now this is only a particular application of the principle that the primary fire is one even in its division. It itself is, even in its unity, both surfeit and want, war and peace (fr. 36). In other words, the "satiety" which makes fire pass into other forms, which makes it seek "rest in change" (fr. 83), and "hide itself" (fr. 10) in the "hidden attunement" of opposition, is only one side of the process. The other is the "want" which leads it to consume the bright vapour as fuel. The upward path is nothing without the downward (fr. 69). If either were to cease, the other would cease too, and the world would disappear; for it takes both to make an apparently stable reality.
All other utterances of the kind are to be explained in the same way. If there were no cold, there would be no heat; for a thing can only grow warm if, and in so far as, it is already cold. And the same thing applies to the opposition of wet and dry (fr. 39). These, it will be observed, are just the two primary oppositions of Anaximander, and Herakleitos is showing that the war between them is really peace, for it is the common element in them (fr. 62) which appears as strife, and that very strife is justice, and not, as Anaximander had taught, an injustice which they commit
